1. Field of the Invention
The invention relates to a nitride semiconductor free-standing substrate, a method of manufacturing the same and a nitride semiconductor device, in more particular, relates to a nitride semiconductor free-standing substrate used for electronic devices such as a light-emitting element which emits blue light, green light or ultraviolet light, a method of manufacturing the same and a nitride semiconductor device.
2. Description of the Related Art
In recent years, a free-standing substrate formed of GaN single crystal which is a nitride semiconductor single crystal with low defect density has been provided by various manufacturing methods, and a semiconductor laser using a nitride semiconductor is in practical use. As a method of manufacturing a free-standing substrate formed of GaN single crystal, for example, a method in which GaN is grown thick on a seed substrate by Hydride Vapor Phase Epitaxy (HVPE) method and the seed substrate is removed during or after the growth, Na flux method in which a Ga metal is mixed into molten Na and pressure is subsequently applied to the entirety thereof by nitrogen for precipitating GaN on a seed crystal, high-pressure synthesis method for synthesizing GaN directly from Ga and nitrogen under the high temperature and high pressure, ammonothermal synthesis in which Ga or GaN is dissolved in ammonia for precipitating GaN on a seed crystal under the lower temperature and lower pressure than the high-pressure synthesis method, and sublimation method for synthesizing GaN from Ga vapor and ammonia, etc., are known.
Among these various manufacturing methods, certain manufacturing methods using the HVPE method are currently most successful, and GaN free-standing substrates with large area (with a diameter of 2 inches) manufactured by these methods have been already commercially available. For example, a method in which Ti is deposited on a surface of a GaN thin film on a sapphire substrate and heat treatment is applied for forming a void on the surface of the GaN thin film, GaN is grown thick thereon by the HVPE method and the sapphire substrate is subsequently removed from the void portion (Void-Assisted Separation: VAS method, e.g., see a non-patent literary document of Yuichi Oshima et. Al. Japanese Journal of Applied Physics, Vol. 42 (2003) pp. L1-L3) is known. In addition, a method in which a surface of a GaAs substrate is partially covered by an insulator mask, GaN is grown thick on the insulator mask using the HVPE method and the GaAs substrate is subsequently removed (DEEP method, e.g., see a non-patent literary document of Kensaku Motoki et. al., Journal of Crystal Growth Vol. 305 (2007) pp. 377-383) is known.
In addition, it is reported that, when hardness of high-pressure synthesized GaN microcrystal (3 mm×2 mm×0.3 mm) was measured by nanoindentation method, the hardness of 20 GPa was obtained (e.g., see a non-patent literary document of R. Nowak et. al., Applied Physics Letters Vol. 75 (1999) pp. 2070-2072). In addition, it is reported that GaN single crystal is high-pressure synthesized and a carried concentration in GaN is adjusted to 5×1019/cm3 (e.g., see a non-patent literary document of K. Saarinen et. al., Physical Review Letters Vol. 79 (1997) pp. 3030-3033).
In addition, a nitride semiconductor single crystal substrate, which has a composition of AlxGa1-xN (0≦x≦1), a fracture toughness value of (1.2-1.7x) MPa·m1/2 or more and a dimension of 20 cm2 or more, is known (e.g., see JP-A 2006-44982). In the nitride semiconductor single crystal substrate described in JP-A 2006-44982, since fracture toughness has been improved, it is possible to suppress destruction of the nitride semiconductor single crystal substrate and to improve productivity in a manufacturing process of a semiconductor electronic device using the nitride semiconductor single crystal substrate.